Isaiah also cries concerning Israel, Though the number of the children of Israel be as the sand of the sea, a remnant shall be saved:
Read Chapter 9
Paul says this because Isaiah was crying out for those who would believe in Christ. It is these who are the true Israel…. The others have gone away from the law because they have not believed in him whom the law promised would alone be sufficient for salvation. Therefore they became apostate, because by not accepting Christ they became lawbreakers. Therefore, of that great number only those who God foreknew would believe have been saved. Commentary on Paul’s Epistles.
If by remnant …we are to understand not election of the justified to eternal life but election of those who are to be justified, that kind of election is truly hidden and cannot be known by us, who must regard all men as parts of a single lump of clay. If some claim to be able to know it, I must confess my own weakness in this matter. .
This shows that the Lord is the cornerstone, uniting both walls in himself. Hosea’s testimony is spoken of the Gentiles, but the Lord unites both Jews and Gentiles, according to what he said in the gospel about the latter: “I have other sheep that are not of this fold; I must bring them also and they will heed my voice. So there shall be one flock, one shepherd.”